The present invention relates to a friction material designed for fitting to a device using friction in a liquid medium, and the method of producing such a friction material and the device to which it is fitted.
More particularly, such a friction material takes the form of a flat ring or a truncated cone and the device to which it is fitted is a clutch or brake disc, notably for an automatic gear box or associated therewith, operating in oil, or a synchronisation ring or cone for a manually-operated gearbox also operating in oil, such a device being installed in a vehicle.
The friction materials used up to now for the aforementioned applications are of three types: materials of the paper type, sintered materials and graphite-containing moulded materials.
The materials of the paper type consist essentially of cellulose fibres impregnated with resin.
Such materials are obtained by a wet method using a normal paper-making process, that is to say by dispersing cellulose fibres in an aqueous solution containing a resin, then spinning and drying.
Such a method necessarily involves using short fibres, with an average length below one millimeter.
Materials of this type have the drawback of degrading very rapidly as soon as their temperature reaches 150xc2x0 C., which is the case when the device that is equipped with the friction material must, within a small space, transmit or absorb high torques at speeds which, in practice, are growing ever higher.
This situation now arises by virtue of, on the one hand, the increasing power of thermal engines and, on the other hand, the reduction in the size of the devices for transmitting engine torque, which make it necessary to increase the gripping pressure of the friction devices.
Sintered materials do not exhibit the above described drawback but, unlike with materials of the paper type, the coefficients of friction obtained are low.
Moreover, these materials generate damaging vibrations and noises.
Materials of the graphite-containing moulded type have a relatively high cost price and do not permit stable transmission of a torque.
The aim of the invention is to overcome the aforementioned drawbacks by proposing a friction material for a liquid medium which has in particular a high, stable coefficient of friction, a high resistance to heating at high working pressures, and good resistance to wear.
A friction material for a liquid medium, according to the invention, is characterized in that it consists of a mat of fibres impregnated with a thermosetting resin, and in that the fibres have a length of at least 12 mm.
According to other characteristics taken separately or in combination:
the average length of the fibres is at most 120 mm;
the fibres are chosen from the group of fibres of glass, wool, cotton, ceramic, polyacrylonitrile, preoxidized polyacrylonitrile and aramid;
fillers in powder form are incorporated into the mat, comprising all or some of the following elements or compounds: copper, rockwool, carbon (coke and/or reduced-powder carbon fibres, graphite), zirconium silicate, iron sulphide, alumina, rubber and diatoms;
fillers in the form of pulps are incorporated into the mat, comprising all or some of the following compounds: pulps of glass, aramid, acrylic and phenolic fibres;
the resin of the thermosetting type includes a polar solvent, preferably aqueous;
the thermosetting resin has latex and/or fillers in powder form added to it which comprise all or some of the following elements or compounds: copper, rockwool, carbon (coke and/or reduced-powder carbon fibres, graphite), zirconium silicate, iron sulphide, alumina, rubber and diatoms.
The method of producing the friction material according to the invention is characterized by the following steps:
a) a mixture of fibres of the same nature or of different natures as defined above is produced in a mixer;
b) the mixture is carded to form a card web;
c) the card web is lapped;
d) the lap thus formed is needled;
e) the needled mat is impregnated with a thermosetting resin; and
f) the impregnated mat is dried.
According to other characteristics taken independently or in combination:
between steps b) and c) above, fillers in powder form as defined above are sprinkled on the card web;
before step e) the thermosetting resin has fillers as defined above added to it;
step e) is preceded by an operation of impregnation of the needled mat by means of a dilution or dispersion in a liquid of the fillers as defined above;
the carding is effected by means of a wool-type card;
the needling operation is preceded by a preliminary needling operation;
the resin impregnation is effected by soaking in a tank containing the resin in solution or dispersed in water; drying is preceded by a squeezing or hydroextraction operation;
after or during drying, the mat is wound up.
As a variant, the method is characterized by the following operations;
a) a mixture of fibres of the same nature or of different natures, as defined above, is produced in a mixer;
b) the mixture is carded to form a card web;
c"") fillers in powder form as defined above, and a resin in powder form, are sprinkled on the card web;
d"") the mat is pressed while being brought to an appropriate temperature to ensure the flow of the resin.
In order to produce a device coated with friction material, the method according to the invention is as follows:
g) a ring, or as a variant a plurality of sectors forming a ring, is cut out from the mat produced as indicated above;
i) the ring or plurality of sectors forming a ring is placed in the bottom of a mould;
j) a metal support is placed in the mould on the ring or on the plurality of sectors forming a ring;
k) where appropriate, a second ring or a plurality of sectors forming a ring is placed on the metal support, opposite the ring or the plurality of sectors forming a ring;
l) the mould is closed, shims being disposed so as to control and limit the movement of a piston closing the mould;
m) heating under pressure is effected in the mould, thereby also ensuring the adhesion of the ring, and where applicable of the second ring, to the metal support;
n) the mould is opened and the device covered with the friction material is cooled.
As a variant, step g) is replaced by a step h) identical thereto, but conducted between steps d) and e) above.
According to other characteristics of the invention, taken independently or in combination:
the mould and piston have a flat bottom;
the bottom of the mould is grooved;
the mould and the piston are in the shape of a truncated cone;
the shims limiting the movement of the piston are sized so that the porosity of the friction material is between 20% and 70%;
the heating temperature is between 130xc2x0 C. and 220xc2x0 C.
Other characteristics and advantages of the product and of the method will appear from a reading of the description that follows, of example embodiments and implementations of the invention, in relation to the accompanying drawings.